Movie gun a good replica of a 'Pedrero'
I found my original copy of the National Park Service publication "Artillery Through The Ages"* , and can now state with good confidence that our movie "cannon" is what was called, in the Spanish Ordnance terminology of the time, a "Pedrero", a bronze gun specifically for firing stone cannonballs.
The prop cannonballs have the right appearance for stone cannonballs, down to the inset lifting eyes that are prominent in the Avila sequence when the cannonballs are first dragged toward the gun.
The movie gun has unusually large proportions in the breech, no doubt to make it appear more impressive, but it's overall proportions are correct for Pedrero's, i.e., has a very large bore for big projectiles, in proportion to the barrel length and thickness. The largest real "pedreros" of Spanish record would have fired 50-pound balls. Our movie gun is made to a more impressive size, about the same caliber as Scotland's famous "Mons Meg" bombarde in Edinburgh, for about a 330 pound stone ball.
Real guns this size were slow to fire, about once per hour (real stone cannonballs that heavy would be lifted to the gun with a crane identical to the one used to lift the gun carriage off its trailing wheels in the film), but realism here would only be appropriate in a documentary, not an adventure-spectacle. The cannon firing sequence had to be rapid to fit the pace of the conclusion, so for the film it made sense to simply have two men quickly lift the cannonballs to the gun, and assign a fictitious weight of "96 pounds" to the cannonballs, a reasonable weight for two men to lift. Since the average spectator has no knowledge of such details, the technical inaccuracy does no harm to the story.
I've started work on a model of the Movie Gun at 1-inch = 1-foot scale, and have started a web page for the same at (copy and paste the link to your browser if you are interested):
http://www.shivakalpa.org/johnp/cannons/
Once I'm reasonably satisfied with the overall accuracy of the model's dimensions, I plan to publish the plans (Free) on the site for other model-builders (as my dimensions are taken manually from the screen, they will not be 100 per-cent accurate).
* available from booksellers, but also viewable (free) online from the NPS at:
http://www.nps.gov/history/history/online_books/source/is3/index.htm